1. Field of the Invention
This invention relates to elastomeric or resilient test cups of the type which are positioned internally of tubing to facilitate pressure testing of tubing. The cups enable the establishment of axially spaced seals within the tubing for the purpose of confining a pressurized fluid. More particularly, the invention relates to resilient test cups currently used in association with a testing tool, including a mandril upon which the test cups are located, for testing oil and gas well tubing when the assembled cups and tool are positioned internally of the tubing to be tested.
2. Brief Description of the Prior Art
It is desirable to relatively frequently test tubing used in the drilling of oil and gas wells, and in the production of hydrocarbons from such wells, in order to determine whether the tubing has become fatigued, or has developed pinhole leaks or has in some other way been weakened so as to decrease its capability of satisfactorily conveying high pressure fluids. Pressure testing of tubing by the use of internally applied fluid pressures is one testing technique widely used to determine the continuing integrity, or lack of it, of tubing sections used to make up a downhole string.
Previously, one widely practiced form of pressure testing of tubing has employed a pair of axially spaced test cups made of an elastomeric material and mounted on the mandril portion of a test tool inserted in the tubing section to be tested. The test cups are backed against stops carried on the mandril in opposed relation, and fluid is admitted through a tubular portion of the mandril which is perforated to allow the fluid to fill the space between the test cups. As the fluid thus admitted to the space between the test cups is pressured up, the test cups flare out and undergo expansion so as to form a fluid-tight seal with the internal wall of the tubing under test. Continued increase in the pressure of the fluid between the test cups correspondingly increases the pressure on the walls of the tubing under test so that any propensity to fail under the high pressure thus developed is manifested by the failure of the tubing section, or ejection of the test fluid through pinhole leaks or fractures which may have previously developed in the tubing section.
The elastomeric test cups which have been used in the described testing procedure are subjected to the same high pressures as is the tubing under test. The cups, in being restricted in their ability to move apart from each other, are caused to undergo compression as the fluid pressure is increased and the fluid is prevented from bypassing the test cups by the seal which is effected between the outer periphery of the cups and the internal wall of the tubing section under test. The compression of the elastomeric material of which the cup is constructed has often resulted in an earlier destruction or degradation of the test cup than is optimum, thus greatly reducing the number of test repetitions which can be realized during the service life of each cup, and requiring the use of new test cups after several tests have been run to the point of destruction of the cups first used.
One type of elastomeric test cup which has been used in pressure testing of tubing of the type described is a cup which, in form, is an elastomeric sleeve having a relatively thick or heavy body wall between the bore through the sleeve and the external wall thereof. The elastomeric sleeve is provided with a belled or flared end for the purpose of receiving the pressure fluid, and undergoing flaring or expansion to sealingly engage the adjacent internal wall of the tubing under test. This test cup further includes an internal steel sleeve or tubing section which is positioned concentrically inside the belled end of the test cup, and is bonded to a surrounding sheath or tube of the elastomeric material. At its end opposite the belled end, this previously used cup includes an elastomer-embedded steel washer which extends transversely across the test cup and functions as a back-up member which abuts a stop ring secured around a mandril upon which the test cup is slidably mounted. The stop ring functions to limit axial movement of the test cup under the impress of the pressurized fluid. The steel washer is of substantially the same size as the end face of the test cup which is opposite its belled end, and contains a central opening or aperture corresponding in size to the bore through the elastomeric sleeve.
The rubber or other elastomer employed in the prior art test cup under description is generally of two hardnesses, depending upon the elastomer location in the cup. At that portion of the cup which includes the belled end, the elastomer used is made of relatively softer character, and will typically be of 70 durometer hardness. This enables the belled end of the test cup to more easily undergo flaring and expansion at the time when fluid pressure acts upon the cup, and thus facilitates effective sealing between the outer periphery of the belled end of the cup and the internal wall of the tubing under test. The 70 durometer hardness elastomer in the test cup extends over approximately one-half the length of the cup. The opposite end portion of the cup is made of harder elastomer -- typically 90 durometer rubber. It is this portion of the cup which is bonded to the steel washer extending transversely of the cup at its base end opposite the belled end.
In utilizing test cups of the embedded steel washer type for pressure testing tubing, it has been found that such cups are destroyed, or are at least rendered relatively ineffective for test purposes, relatively early in their service life. In a recent pressure test, for example, the cups were fractured and became unsuitable for use so as to require replacement four times during the testing of 1200 feet of tubing sections or joints during an oil well workover. Upon examination of the damaged cups, it was determined that a break or shear plane through the elastomer of the cup existed along a transverse plane extending normal to the axis of the cup at the line of juncture between the relatively soft rubber and the harder rubber. The mating or marriage of the two hardnesses of rubber at this point apparently sets up a plane of weakness within the cup such that fracturing at this location, under the impress of relatively high fluid pressure applied to the cup, occurs relatively easily, and once it has occurred, the susceptibility of the cup to subsequent successful usage is lost.
In another type of test cup which has previously been employed, the steel washer structure located at the end of the test cup opposite its belled end is replaced by a steel cap of U-shaped cross-section. The cap, which is bonded to the rubber in the elastomeric sleeve at this location, has a central hole or aperture in the bottom thereof for registration and alignment with the bore through the elastomeric sleeve. The steel cap employed in this form of test cup includes a cylindrical wall which projects axially toward the belled end of the sleeve over a distance which is about three-quarters of the length of the test cup.
In this type of test cup having a steel cap at one end, all of the elastomer in the sleeve is of uniform hardness and, in order to accommodate the requisite expansion of the belled end of the cup to effect sealing engagement with the tubing under test, is of a relatively soft elastomeric material. Typically, a rubber having a hardness of 70 durometer has been employed.
In the case of this second version of test cup which has previously been used, it has been found that some improvement in the failure rate which is typical and characteristic of the washer-type cup has been realized. Nevertheless, in a shorter time than is desirable, the steel cap-containing test cups must be replaced as a result of fracturing and splitting of the elastomer in the elastomeric sleeve at the point where the metallic cup terminates along the body of the elastomeric sleeve. The reason for fracturing and splitting at this location is not known with certainty, but is believed, from observation, to be the result of the elastomer in the body of the cup forward of the metallic cap undergoing high compression and, in doing so, being forcibly buckled against the end of the metallic cup so as to cause this end of the cap to shear through or cut into the elastomer at this location while the elastomer is under high compression.